Torrefaction is the thermal treatment of biomass in the absence of air at relatively low temperatures from 250° to 300° C. by pyrolytic decomposition.
U.S. Pat. No. 4,347,156 discloses a method for reactivating active carbon in a multiple-hearth furnace, upstream of which there is a drying zone. In said method, a gas flow is diverted from the multiple-hearth furnace and fed to an afterburner for combustion. The exhaust gas that evolves is fed to the drier, which can be operated either as a direct contact-type or as an indirect contact-type heat-exchanger.
Another device for torrefaction is disclosed in WO 2005/056723 A1. In this specification, gases withdrawn from the torrefaction stage are also fed to the drier or recirculated as hot gas into the torrefaction zone.
A device for the decomposition of biomass and for producing a fuel gas is furthermore known from EP 2 017 325 A2; here, the biomass is dried in a drying zone and degassed in a torrefaction zone. Finally, the material so treated is first comminuted and then fed to a pyrolysis stage. Part of the drying gas containing water vapour is discharged from the drier and heated in a heat exchanger and then fed at least partially to the drier again.
US 2010/0083530 A1 describes a method and an installation for the torrefaction of cellulosic material in an inert atmosphere. The material to be treated is fed to a processing chamber comprising a plurality of trays and after treatment is discharged as torrefied material. A steam-laden exhaust gas is discharged from the processing chamber and partially fed via a condenser to a burner, the hot exhaust gas of which is used to heat the remaining part of the steam-laden exhaust gas before the heated remaining part of the steam-laden exhaust gas is recycled to the processing chamber.